Purpose: The purpose of this Core is to develop an infrastructure to standardize, document and formalize existing expertise regarding issues related to conducting research among drug using populations. This Core will serve as a drug user resource for al1 nutrition, endocrine, and metabolic (NEM)-related studies supported by the Center. Services offered by the Core are organized into two main sub-cores: 1) Recruitment and Retention of drug users, and 2) Assessment of sensitive data from drug users. This Core will provide training materials, training sessions, and telephone and in-person consultations for Center Members who wish to learn about or improve their current recruitment and retention strategies in their research studies; or who wish to learn new and optimal methods for soliciting important, but sensitive, information from drug users. The Core has particular expertise in the development and implementation of audio computer-assisted self-interview (ACASI) systems for assessing sensitive risk behaviors.